Help! How can I change the location of db's from /var/imap/db?

[Taro Ikai]

My disk partition assigned to /var is not very big.
One of my __db.*** files filled the disk partition, and 
stopped imapd from accepting any more e-mails.

Can somebody tell me how I can change the location 
of these __db.*** files?

Thanks in advance.

Taro